This project utilizes closed-circuit television in conjunction with microscopy to quantitate small artery (40micra) and vein (80micra) diameters in the bat wing (sub-cutaneous tissue) and rat cremaster (skeletal muscle) during normo- and hypertension. Topical application of norepinephrine, and and isoproternol before and after alpha adrenergic blockade with phentolamin provide assessments of the state of the alpha adrenergic receptors in small blood vessels in subcutaneous and skeletal muscle tissues. Alterations in the inspired concentrations of O 2 and CO 2 allow interpretations about the status of microcirculatory systems for distribution of blood flow within the microcirculation. Comparisons of the effects of topical catecholamines and alterations in inspired O 2 and CO 2 for normo- and hypertensive animals can indicate changes in micro-circulatory control during hypertension. Our recent data suggest the concept that small vein phenomena (post- capillary events) are not involved in he mechanisms of redistribution of blood flow through the capillary networks during hypoxia in a low metabolism tissues such as subcutaneous tissue. That is, small vein behavior is not altered during hypoxia and therefore would not account for any change in precapillary sphincter behavior during hypoxia. This may not be true in high metabolism tissues such as the rat cremaster or in the hypertensive animal. Pharmacological studies have demonstrated the presence of alpha adrenergic, but not beta, receptors in subcutaneous tissues. In addition, the small veins are more sensitive to both epinephrine and norepinephrine than are the small arteries. This differential sensitivity may be lost in the hypertensive animal. At the present time, only preliminary studies with renovascular hypertension in the rat have been completed.